Plant Growth Regulators for Greenhouse Crops
Plant Growth Regulators: Factors Affecting Plant Response
Any factor that affects the rate and quality of plant growth and development can influence a plant’s response to PGRs. All such factors should be considered in a production system. The dosage for specific crops is in most cases presented as a range of values. The decision to work at the top or the bottom of the labeled dosage range must be made by each individual grower. To make this decision, growers must consider all of the factors that affect plant response to PGRs relative to the conditions in their own greenhouses, past experiences with these compounds, and the desired effects.
Plant Factors
Plant Species and Cultivars
Plant species and cultivars vary greatly in growth habits, and in chemical and environmental sensitivity. PGRs that work on one species may not be effective on another specie. Rates that are effective on one cultivar may be too high or too low for another cultivar of the same species.
Stage of Plant Growth
Plants progress through a cycle of growth stages, which include stem extension, internode elongation, leaf expansion, bud formation, flowering opening, and physiological maturity. In general, the rate of physiological development is more rapid under warm conditions with high light than under cool and low light growing conditions over the same period of time.
Plant Vigor
In general, plants with more vigorous growth habits require a higher rate than those with less vigorous growth habits. Avoid using PGRs or use lower rates on less vigorous plants, as the desired effect may be excessive.
Plant Size
Plant size impacts PGR application. Smaller plants require lower rates or less chemical than larger plants. For example, to achieve the same level of control, a plant in a 6 inch pot requires more drench or spray than the same cultivar in a 4 inch pot produced under similar conditions.
Environmental Factors
Temperature and Light
Plants grown under low temperature and light may require a lower dosage or fewer PGR applications than those grown under higher light and temperatures. Adjust concentrations accordingly for winter and summer production, especially when using the more active or stronger PGRs.
Growing Media
Growing medium composition directly affects the efficacy of drench applications of certain PGRs. In general, as organic matter content of a growing medium increases, the effectiveness of a PGR drench decreases.
Water Quality and Amount
Water quality affects the activity of some PGRs. A combination of high pH (>7) and high alkalinity (>100 ppm calcium carbonate equivalent) may reduce the effectiveness of many PGRs. The amount of water in which PGRs are applied can also alter performance.
Physical and Chemical Factors
Residual Chemical Effect
Residual chemical effect refers to the length of time a PGR remains active in the plant after application. Generally, the products that have the shortest-lasting response are those that contain daminozide or chlormequat chloride (e.g., Citadel and Cycocel). Products that contain ancymidol (e.g., Abide and A-Rest) or flurprimidol have relatively moderate strength while those with paclobutrazol (e.g., Bonzi, Paczol, and Piccolo) are often even stronger.
Application Rates
Applications that are made early to a crop (proactive applications) generally need lower rates than those made to crops that are already vigorously growing (reactive applications). The rate used on a plug is usually lower than the rate used on the same species during the finish stage. Rate of application is very dependent on light, temperature, relative humidity, and watering and fertilization practices. Higher rates are needed when temperatures and light levels are high, varieties are vigorous, moderate to high fertility (especially for phosphorus), a warmer day than night temperature, and plants are tightly spaced and/or growing in small containers.
Time of Application
Plant growth regulators are most effective when applied at the appropriate times to regulate plant growth or development. Generally, growth-retarding PGRs should be applied just prior to rapid shoot growth. This is generally one to two weeks after transplanting a plug, after the roots are established, and as the plant resumes active growth. On pinched plants, it is after the new shoots are visible and starting to elongate.
Dosage
Compared to crop protectants (e.g., insecticides and fungicides) PGRs have a relatively narrow acceptable dose range, where overdose can result in negative side effects. Therefore, PGRs are applied in very precise and low concentrations. Extreme care must be taken to mix and apply these chemicals accurately. The crop may be injured if too much chemical is used.
Chemical Uptake and Translocation
Chemical uptake and translocation vary from one PGR to the next. For example, Bonzi, Concise, and Sumagic, (e.g., PGRs with the same active ingredients) are actively taken up by plant roots and are readily transported to shoot tips.
Coverage
Coverage refers to the volume of solution sprayed per unit area, or for drenching, the volume of solution per pot size or area (if applied to soil beds).
Spray Droplet Size
The smaller the average droplet size, the greater the coverage and penetration. However, extremely small drops (i.e., fog) result in drift, take a long time (hours) to settle and may require air circulation to achieve good penetration.
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